LAS VEGAS — It turns out the toughest obstacle of a Tough Mudder-style race might not be dodging live electrical wires, hoisting logs or leaping over a wall of flames. It might be the nasty stomach bug that can come from swallowing the muddy water.

The U.S. Centers for Disease Control and Prevention issued a memo Friday warning that animal feces in the mud along the courses can give participants a bad case of diarrhea. The agency said nearly two dozen people from the Nellis Air Force Base community in Nevada reported coming down sick after participating in a race in rural Beatty, Nev., in October 2012.

Participants make their way up a muddy hill during an endurance race in Henley on Thame on April 26, 2014. (JUSTIN TALLIS/AFP/Getty Images)

The investigation traced the sickness to the bacteria campylobacter coli. It concluded people became ill after accidentally swallowing contaminated water on the course, which was on a cattle ranch within sight of cows and pigs. The sickness generally sets in three days after the race, and it lasts a week.

Adventure races are increasingly popular in the U.S., where they drew about 1.5 million participants in 2012, according to the CDC memo. In the 16-20-kilometre-long Tough Mudder challenges, participants slither on their bellies through fields of mud, plunge into icy water and try to cross lakes while balancing on slippery tightropes.

Participant jump from a four meter high platform into water during an endurance race in Henley on Thames on April 26, 2014. (JUSTIN TALLIS/AFP/Getty Images)

The contests often draw active-duty military personnel or civilians in top shape. But the CDC said those daredevils can be brought to their knees by the stomach bug, which lurks in the animal droppings on the man-made mud fields.

Health officials recommend race organizers warn participants about the gastrointestinal dangers of the contest, and stress the importance of hand-washing and avoiding swallowing the water. They also recommend race organizers set up the courses in areas where animals are less likely to roam.

A participant swims backwards through water underneath a mesh wire fence during an endurance race in Henley on Thames, on April 26, 2014. (JUSTIN TALLIS/AFP/Getty Images)

Officials with Tough Mudder didn’t immediately respond to a message seeking comment Friday morning, and it was unclear whether the organization planned to change any policies in response to the CDC’s findings.

The CDC noted the bacteria causing sickness after races has hit sports events in the past. Campylobacter outbreaks after several bicycle races in Europe were traced to bike tires splashing up dirty water into participants’ mouths.

Participants crawl through mud during an endurance race in Henley on Thames, on April 26, 2014. (JUSTIN TALLIS/AFP/Getty Images)